The present invention relates to screening apparatus which is useful for processing yard waste, wet mulch, and mixtures of soil, rock, stumps, brush and vegetation, possibly at least partially composted, which has been cleared from raw land that is being subjected to development. The apparatus is also useful in recycling efforts aimed at producing wood fuel, coarse mulch, fine mulch and top soil or horticultural soil from raw, unscreened top soil and/or mixed materials resulting from land-clearing operations. Further, the apparatus is useful for processing garbage, municipal solid waste (both fresh and aged), construction and/or building demolition debris, and the raw product of an automobile shredding operation (for separating so-called fluff from shreds of plastic, glass ferrous and non-ferrous metals and like).
Such devices typically include a prescreening separator, for removing large rocks, stumps, roots and unchipped brush from the processing stream. Overflow is rejected and underflow passes to a tilted or generally horizontal screening drum. Drum overflow is collected as a coarse fraction, drum underflow is collected as a desirable product for use as is, or after further processing. (In the course of some waste-stream processing, neither underflow nor overflow may have a high value, but the separation is desirable and beneficial nevertheless, because the underflow and overflow products may be able to be disposed of to different repositories neither of which economically (or, perhaps legally) could accept the mixed waste stream.)
Frequently, the conventional screening drum has as its radially outer peripheral wall, a perforated metal wall, or a set of longitudinal bars wrapped by a set of circumferential rings. Some prior art drums have a composite peripheral wall, e.g., a foraminous metal screen wrapped by a set of longitudinal bars and/or a set of circumferential rings. In some prior art instances, some of the bars are movably mounted to drum end walls, in order to permit bar shifting for unclogging. Nevertheless, a persistent shortcoming of prior art screening apparatus of the type referred to above, is excessive clogging. As a result, the apparatus must be tended by too many operators, limiting its chances for profitable operation. Clogging causes excessive downtime for performance of unclogging operations and repairs (when jams cause parts to break). Other typical shortcomings of conventional screening apparatus of this type include lack or difficulty of portability, inability to regulate feeding so that the apparatus operates at full capacity without overloading, clogging of prescreening devices, and inability to operate on wet and/or partially frozen material.